Film laminate food wrap and food pouch therefrom

ABSTRACT

A flexible film laminate of (a) a polyacrylonitrile film containing a heterized ethylene oxide-propylene oxide copolymer; (b) contacting said film with an adhesive combination layer of a styrene-butadiene block copolymer and a terpolymer of 1,3-pentadiene, a monocyclic terpene and alpha methylstyrene; and (c) a second polyacrylonitrile film or a heat-sealable polyolefin film contacting said adhesive layer; a product pouch made therefrom; and a method of forming said laminate.

BACKGROUND OF THE INVENTION

The present invention relates to a flexible, boil resistant filmlaminate and to packages, for example, retortable food pouchesfabricated therefrom.

Certain packaging applications, for example, retort packages, requirethat the packaging material constitute a good barrier to the passage ofoxygen and moisture vapor. In addition, the packaging of certain foodsrequire that a minimum of oxygen or air are transmitted to the contents.Cheese is one food product which should be packaged to the exclusion ofoxygen.

Oriented polyacrylonitrile (PAN) film has excellent oxygen barrierproperties. Its moisture barrier properties, however, are less thandesirable for certain packaging purposes. Furthermore, it is notsealable to itself. Both deficiencies can be overcome by applying to onesurface of the PAN a layer of a thermoplastic material having goodmoisture barrier properties. For some purposes it is desirable to bondthe PAN film to itself. It is necessary, therefore, to consider theimposition of an adhesive or bonding system between either two layers ofPAN or a layer of PAN and a thermoplastic moisture barrier film. Byproviding an effective adhesive bond between such films the result willbe satisfying a need that has developed in the food packaging industry.In recent years, the industry has shown intense interest in the conceptof pouch-packaged foods which, among other advantages, do not requirefreezing for their preservation and can therefore dispense with costlyand energy intensive refrigerated transportation and storage facilities.Much effort has gone into the development of a flexible food pouch whichnot only can withstand the rigors of sterilization and later reheating,e.g., microwave heating, and provide various properties which aresufficient to adequately protect the contents during storage, but whichalso employ materials which are toxicologically safe.

In accordance with U.S. Pat. No. 3,453,173, apolyolefin-polyacrylonitrile laminate, which is said to possess superiorheat-sealed strength and excellent barrier properties to thetransmission of gases and is adaptable for the construction of foodcontainers, is prepared by bringing the polyolefin surface and thepolyacrylonitrile surface together and adhering them to each otherthrough an adhesive. Either or both surfaces can be pre-treated in somemanner in an effort to make them more adherent, e.g., by treatment witha gaseous mixture of boron trifluoride, as described in British Pat. No.834,196; by flame treatment or by treatment by corona discharge, asdescribed in U.S. Pat. No. 2,632,921. Among the adhesives employed inthe manufacture of the laminate is an ethylene vinylacetate copolymerdescribed in U.S. Pat. No. 2,200,429. Because the conditions ofprepartion and the adhesive contemplated are not conducive to theformation of primary valence bonding, the interlaminar adhesion inboiling water would not be expected to be strong. Resort also has beenhad to metalizing the PAN film in order to employ the metal layer as aneffective surface through which adhesion can be accomplished to thewater vapor barrier film.

It is an object of the present invention to present polyacrylonitrilefilm laminates or composites of two or more PAN layers bonded togetherwith a unique and effective bonding system.

It is an object of the present invention to present polyacrylonitrilefilm laminates or composites of PAN and a thermoplastic moisture barrierlayer bonded together with a unique and effective bonding system.

It is a further object to present a food or product pouch or tube madefrom said laminate.

It is yet another object of the invention to present a method forimproving the bond strength between films of the type described above.

SUMMARY OF THE INVENTION

In accordance with the present invention, a flexible film laminateadaptable for use in the packaging of a commodity and in particularfood, is provided which comprises:

(a) an oriented polyacrylonitrile film containing an adhesion promotingproportion of a liquid heterized ethylene oxide-propylene oxidecopolymer prepared by reacting said oxides with at least one monohydricalcohol active hydrogen compound initiator having from about 1 to about10 aliphatic carbon atoms;

(b) an adhesive layer contacting said polyacrylonitrile film, saidadhesive layer being a combination of (1) a styrene-butadiene blockcopolymer having a monomer parts by weight ratio of from about 1:3 toabout 3:1 and (2) a random terpolymer of from about 30 to about 40 partsby weight of 1,3-pentadiene; about 30 to about 40 parts by weight of amonocyclic terpene and from about 25 to about 35 parts by weight ofalpha methylstyrene, said combination consisting essentially of fromabout 1 to about 3 parts by weight of the block copolymer to from about3 to about 1 part by weight of the terpolymer; and

(c) either a second oriented polyacrylonitrile film containing saidheterized copolymer or a heat-sealable polyolefin, contacting saidadhesive layer.

The process for adhesively bonding together two or morepolyacrylonitrile films or a polyacrylonitrile film and a heat-sealablepolyolefin film comprises: incorporating into the polyacrylonitrile filman adhesion promoting proportion of a liquid heterized ethyleneoxide-propylene oxide copolymer prepared by reacting said oxides with atleast one monohydric alcohol active hydrogen compound initiator havingfrom about 1 to about 10 carbon atoms and applying pressure to the filmsurfaces having an interposed layer of a composition comprising acombination of (1) a styrene-butadiene block copolymer having a monomerparts by weight ratio of from about 1:3 to about 3:1 and (2) a randomterpolymer of from about 30 to about 40 parts by weight of1,3-pentadiene; about 30 to about 40 parts by weight of a monocyclicterpene and from about 25 to about 35 parts by weight of alphamethylstyrene, said combination consisting essentially of from about 1to about 3 parts by weight of the block copolymer to from about 3 toabout 1 part by weight of the terpolymer.

DETAILED DESCRIPTION OF THE INVENTION

The polyacrylonitrile film of the present invention comprises a highnitrile addition polymer containing at least 80% acrylonitrile repeatingunits. Flat PAN film may be solvent cast according to the process ofU.S. Pat. No. 4,066,731 (which is incorporated herein by reference)wherein acrylonitrile homopolymer or interpolymer is cast onto arotating drum from a sheeting dye and coagulated as a self-supportingfilm. Organic solvent in said film, such as dimethyl sulfoxide, can beexchanged therefrom by a water bath to obtain an aquagel film typicallycontaining 40 to 60% water integrally bound in the molecular intersticesor dispersed in the orientable polymer matrix. A tubular PAN film can beextruded and water-coagulated, if desired, and the unoriented film canbe slit and fed to the treatment and orientation units as a flat strip.Aqueous PAN film can also be made by melt-extrusion of a hightemperature polymer hydrate in a known manner.

The preferred PAN feedstock is an addition polymer material containingsufficient water to be stretched at low temperatures. Acrylonitrilepolymers containing at least 5% H₂ O, preferably aquagels, containingabout 40 to 60% H₂ O, are excellent film substrates for use herein. Thepresent system is especially valuable for treating PAN homopolymer suchas du Pont Type A resin. Homopolymer PAN, copolymers and interpolymerswith hydrophilic ethylenically-unsaturated monomers, such as acrylicacid and esters, etc., may be adapted to the novel process. Typicalhigh-nitrile polymers are disclosed in U.S. Pat. Nos. 2,585,444;3,873,508; 3,896,204; 3,984,601; and 4,053,442, incorporated herein byreference.

Incorporated into the polyacrylonitrile film as a plasticizer and as anadhesion promoting agent is the above described liquid polyether. Thesematerials are water soluble or water miscible and have an averagemolecular weight of from about 500 to 5000 and a Saybolt viscosity at100° F. of from about 85 to about 5200 seconds. The preferred averagemolecular weight is from about 800 to about 4600 and a preferred Sayboltviscosity at 100° F. is from about 160 to about 5100 seconds. Aparticularly preferred average molecular weight is from about 800 toabout 2000. The polyether is present in the film in a plasticizing andadhesion promoting proportion. This amount should be at least 1% of thePAN polymer. More specifically, the polyether is present in from about 5to about 20 weight percent of the PAN polymer.

A process for forming the PAN film having the liquid or polyetherincorporated therein comprises the steps of: (a) maintaining apredetermined concentration, in an aqueous solution, of said liquidpolyether, said polyether having been prepared generally as indicatedabove: (b) contacting continuous polyacrylonitrile aquagel film withsaid solution, thereby including said polyether into the aquagel; (c)sequentially or simultaneously stretching the polyether-aquagel-PAN filmsystem to orient the polyacrylonitrile film; and (d) drying the orientedfilm to at least substantially remove water and yield the polyetherimbibed film.

This process is particularly adaptable for imbibing PAN with the subjectwater-miscible polyether by maintaining a predetermined concentration ofthe polyether in the aqueous bath under steady state process conditions.The continuous polymeric aquagel film is passed through the bath atsubstantially constant water content. The polyether impregnant isthereby coated onto the film. The impregnant may be added to the aqueousbath in a steady stream comprised essentially of the polyether. Thepolyether can be metered to the bath substantially undiluted andadditional water can be input to the process as needed. It is to beunderstood that the polyether alone, or as a water solution, can beapplied to the PAN film in any fashion, for example, by spray, roller,brush, etc., application.

The polyethers contemplated herein have been known since at least the1940's. They are essentially mixtures of monohydroxy alcohols which maybe obtained by the addition to one or more monohydroxy aliphaticalcohols, of a mixture of alkylene oxides, containing ethylene oxide and1,2-propylene oxide, in an oxide ratio of from about 75-25 to 10-90ethylene oxide to 1,2-propylene oxide. Stated otherwise, the oxide ratiocan be from 1/3 part to 9 parts propylene oxide per part of ethyleneoxide by weight. These ratios are qualified by the molecular weightrange mentioned above and the necessity for at least substantial watersolubility or miscibility.

The reaction which takes place between the alcohol or alcohols and theethylene oxide and the 1,2-propylene oxide, seems to be a simpleaddition reaction wherein the alkylene oxide molecules undergoconversion to the corresponding oxyalkylene radicals as illustrated forany given molecule by the following general equation:

    ROH+y(C.sub.2 H.sub.4 O)+z(CH.sub.3 C.sub.2 H.sub.3 O)→R(OC.sub.n H.sub.2n).sub.x OH

wherein ROH is one or more aliphatic monohydroxy alcohols; x and zrepresent the mols of ethylene oxide and 1,2-propylene oxide,respectively; n is both 2 and 3 in a single molecule, the total numberof times n has a value of 2 can be equal to y and the total number oftimes n has a value of 3 can be equal to z; and x is the total number ofsuch oxyalkylene groups, being equal to y+z.

From such properties and the average molecular weight, refractive index,density, viscosity, rate of change of viscosity with change intemperature, as well as theoretical considerations, it appears thatthese products are complex mixtures of monohydroxy polyoxyalkylenealiphatic monoethers having polyoxyalkylene chains of different lengthsin different internal configurations with the hydroxyl group appearingat one end of the chain and the aliphatic group of the starting alcoholor starting alcohols at the other, and containing in a single molecule,both the oxyethylene group and the oxy-1-2,propylene group. Thesematerials have been referred to as mixtures of monohydroxy heterizedoxyethylene oxy-1,2-propylene aliphatic monoethers. By the term"heterized", it is meant that the monoethers vary in internalconfiguration from molecule to molecule, such variation arising out of arandomness of the distribution of the oxyethylene and theoxy-1,2-propylene groups therein, for instance, from the concurrentreaction of ethylene oxide and 1,2-propylene oxide with the aliphaticmonohydroxy alcohol or alcohols. By the use of the term "heterized" isnot meant to exclude the presence of relatively small blocks ofpolyethylene oxide groups and with relatively small blocks ofpolyoxy-1,2-propylene oxide groups. These materials have also beengenerically referred to as polyalkoxylated polyethers apparently when amixed aliphatic alcohol initiator is employed in the formation thereof.

For more detailed exposition of the materials contemplated herein andthe process of preparing the same, reference is made to U.S. Pat. No.2,425,755 and copending application Ser. No. 331,427 filed Dec. 16, 1981which are incorporated herein by reference. Particularly preferred foruse herein as an adhesion enhancer and plasticizer for the PAN are themixed polyethers formed with the intiator, butyl alcohol, or a mixtureof ethyl alcohol and butyl alcohol.

As indicated above the adhesive system of the present invention can beemployed to bond polyacrylonitrile film to another polyacrylonitrilefilm or to a thermoplastic heat-sealable polymer film. Particularlypreferred are polyolefins. The polyolefins which are useful hereininclude the film-forming homopolymers and copolymers formed by thepolymerization of one or more C₂ -C₈ alpha-olefins, for exampleethylene, propylene, butene-1, pentene-1, hexene-1, heptene-1, octene-1,etc. The useful polyolefins also include film-forming copolymers of oneor more of the alpha-olefins, copolymerized with up to 20 weight percenttotal, and preferably not more than 5 weight percent total of one ormore monomers copolymerizable with the alpha-olefin. Examples of suchmonomers include, carbon monoxide; sulfur dioxide, acrylic acid or/andacrylate ester such as methyl acrylate, amyl acrylate, hexyl acrylate,cyclohexyl acrylate, and the like; methacrylic acid or a methacrylateester, such as methyl methacrylate, ethyl methacrylate, propylmethacrylate, butyl methacrylate, amyl methacrylate, and the like; vinylacetic acid or a vinyl ester such as vinyl formate, vinyl acetate, vinylpropionate, vinyl butiorate, vinyl benzoate, isopropynol acetate and thelike.

Procedures for the polymerization of alpha olefins and thecopolymerization of alpha olefins with other monomers such as thoserecited herein are well-known and do not constitute a part of thisinvention. The polyolefins which are useful herein can be a atactic,isotactic, syndiotactic, crystalline or amorphous or a combination ofany of the foregoing. The preferred polyolefin films are thehomopolymers and the copolymers of ethylene and propylene. Aparticularly preferred polyolefin film is a random copolymer ofpropylene with a minor amount of ethylene (commercially available fromExxon Corporation as EX-24). Good adhesion can be obtained withoutsubjecting the films of the present invention to a surface treatment,however, better adhesion is obtained by subjecting the polyolefin filmsto corona discharge and/or by other equivalent means prior toapplication of the adhesive.

The adhesive system of the present invention not only must be aneffective adhesive to the PAN film but under the conditions of thepresent invention it also must be an effective bonding system to thethermoplastic moisture barrier film.

For reasons not completely understood, the presence of the heterizedpolyether in the PAN film causes a significantly stronger bond at theinterface between the PAN and the adhesive system than would be possibleif the heterized polyether were not present in the PAN.

The adhesive system is a combination of a commercially available productsold under the name Kraton 1000 series products (Shell Chemical Company,Polymers Division) and a second component. One material of this series,Kraton 1102 is a styrene-butadiene block copolymer having the followingproperties determined at about 23° C. on films cast from a toluenesolution: tensile strength, psi 4600; 300% Modulus, psi 400; Elongation%, 880; Hardness, Shore A 62; Angle Tear Strength-Die C, pli 190;Specific Gravity 0.94; and Solution Viscosity-25% w, cps from a toluenesolution, 1200.

The second component of this adhesive combination is a resinous randomterpolymer which contains from about 30 to about 40 parts by weight of1,3-pentadiene; about 30 to about 40 parts by weight of a monocyclicterpene; and about 25 to about 35 parts by weight of alphamethylstyrene. Although a preferred monocyclic terpene for employment inthe present combination is beta-phellandrene, other monocyclic terpenes,or mixtures thereof, may be employed. These include dipentene,alpha-terpinene, beta-terpinene, alpha-phellandrene, gama-terpinene,terpinolene, sylvestrene, origanene, the pyronenes and the like.

It is preferred that the terpolymer additive have the followingproperties:

Melting point range (ring and ball) 75°-125° C.;

Specific gravity 1.08-0.90;

Bromine number 6-14;

Iodine number 79;

Acid value, less than 1;

Saponification number, less than 1;

Decomposition temperature (in air) 300° C.;

Color (in 50% toluene solution; Gardner 6; and

Viscosity (in toluene) 70%.

A particularly preferred random terpolymer is prepared by polymerizing afeedstream containing 37.5% by weight 1,3-pentadiene, 37.5% by weightbeta-phellandrene, and 25% by weight of alpha-methylstyrene in a toluenediluent in the presence of an aluminum chloride catalyst. The resultingterpolymer contains 34.0 parts by weight of 1,3-pentadiene, 36.5 partsof beta-phellandrene and 29.5 parts by weight of alpha methylstyrene.This material has a molecular weight (weight average) of 1880, a brominenumber of 14, an iodine number of 79, a glass transition temperature of38° C., a viscosity in toluene, f to g, of 70% and a decompositiontemperature (in air) of 300° C.

The individual films comprising the laminate described herein can beprepared in widely varying thicknesses, for example, from about 0.1 milsto about 10 mils and preferably from about 0.5 mils to about 5 mils. Thethickness of the adhesive layer should, of course, be no thicker than isrequired for the intended utility. In some instances the thickness willbe in the micron range whereas for other purposes the thickness may bein the range a fraction of a mil to more than 1 mil.

The layers which comprise the laminate of the present invention can beassembled in a variety of ways. For example, the combination adhesive,styrene-butadiene block copolymer mixed with the random terpolymer, canbe deposited on the PAN film out of a suitable solution, e.g., a toluenesolution. Thereafter a second PAN film can be brought into interfaciallamination with the adhesive film and this contact can be facilitated bythe appropriate use of pressure and/or heat. Alternatively, the two PANfilms can be separately covered or coated with the adhesive and thenbrought together by the use of pressure and/or heat. When the secondlayer to be laminated is a thermoplastic film of the type describedabove, the combination adhesive can be applied to this layer out of asolvent solution and the PAN film can, in any appropriate manner, bebrought into intimate interfacial contact with the appropriate use ofpressure and/or heat.

After the layers of the present invention have been laminated together,they can be fashioned into an appropriate container. For example, theycan be fabricated into retortable pouches employing known methods. Inaccordance, with one such procedure, the laminate can be folded overupon itself with the heat-sealable polyolefin sides facing each otherand these faces can be edged sealed between heated plates. Such a pouchcan be filled with food or other material, the top heat sealed and aneffective retortable pouch is obtained. In the instance, where twolaminated polyacrylonitrile films are employed, the edge sealing can beaccomplished by use of the adhesive of the present invention since thepolyacrylonitrile surfaces cannot be heat sealed together.

EXAMPLE

A strip of corona treated, 3 mil thick film, of a random copolymer ofpropylene with a minor amount of ethylene (commercially available fromExxon Corporation as EX-24) is coated with a toluene solution containing25% by weight solids of a combination containing 6 parts by weight of astyrene-butadiene block copolymer and 13 parts by weight of a terpolymerof 34 parts by weight of 1,3-pentadiene, 36.5 parts by weight ofbeta-phellandrene and 29.5 parts by weight of alpha-methylstyrene. Thislayer is dried at about 200° F. for about 20 seconds. A correspondingstrip of biaxially oriented polyacrylonitrile homopolymer film ofapproximately 0.65 mil thick is prepared said film containing about 14%by weight of a polyether within the scope of the polyethers definedherein. This polyether is identified as Pluracol® W-660, available fromBASF Wyandotte Corporation. This strip is applied to the adhesive coatedcopolymer of propylene and ethylene. These laminated films are pressedtogether under moderate pressure at least sufficient to cause asubstantially uniform interfacial contact there between. The laminate isthen heated at about 250° F. for about 15 seconds. This will yield alaminate which can be peeled apart but only with significant force. Thelamination of polyacrylonitrile film not containing the heterizedpolyether of the present invention to another PAN film, or to apolyolefin film with, the adhesive of the present invention will notresult in as strong and effective a laminate as with PAN containing theheterized polyether.

What is claimed is:
 1. A flexible film laminate comprising:(a) anoriented polyacrylonitrile film containing an adhesion promotingproportion of a liquid heterized ethylene oxide-propylene oxidecopolymer prepared by reacting said oxides with at least one monohydricalcohol active hydrogen compound initiator having from about 1 to about10 aliphatic carbon atoms; (b) an adhesive layer contacting saidpolyacrylonitrile film, said adhesive layer being a combination of (1) astyrene-butadiene block copolymer having a monomer parts by weight ratioof from about 1:3 to about 3:1 and (2) a random terpolymer of from about30 to about 40 parts by weight of 1,3-pentadiene; about 30 to about 40parts by weight of a monocyclic terpene and from about 25 to about 35parts by weight of alpha methylstyrene, said combination consistingessentially of from 1 to about 3 parts by weight of the block copolymerto from about 3 to about 1 part by weight of the terpolymer; and (c) asecond oriented polyacrylonitrile film or a heat-sealable polyolefinfilm contacting said adhesive layer.
 2. The laminate of claim 1 whereinsaid polyacrylonitrile contains at least 80% acrylonitrile repeatingunits.
 3. The laminate of claim 1 wherein said polyacrylonitrile is ahomopolymer.
 4. The laminate of claim 1 in which the adhesive layercontains from about 6 parts by weight of said styrene-butadiene blockcopolymer and about 13 parts by weight of said terpolymer.
 5. Thelaminate of claim 4 wherein said terpene is beta-phellandrene.
 6. Thelaminate of claim 1 wherein said ethylene oxide-propylene oxidecopolymer is water soluble or water miscible and has an averagemolecular weight of from about 500 to about 5,000 and a Sayboltviscosity at 100° F. of from about 85 to about 5200 seconds.
 7. Thelaminate of claim 6 wherein said monohydric initiator is one or moremembers selected from the group consisting of methanol, ethanol,proponal and butanol.
 8. The laminate of claim 1 wherein said ethyleneoxide-propylene oxide copolymer is present in at least 1 weight percentof the polyacrylonitrile film.
 9. The laminate of claim 8 wherein saidethylene oxide-propylene copolymer is present in from 5 to about 20weight percent of the polyacrylonitrile.
 10. The laminate of claim 1wherein said heat-sealable polyolefin film is a homopolymer or acopolymer of a C₂ -C₈ alpha-olefin.
 11. The laminate of claim 10 inwhich the polyolefin film contains up to about 20 weight percent of oneor more monomers copolymerizable with the alpha-olefin.
 12. The laminateof claim 11 wherein the polyolefin film contains not more than about 5weight percent of one or more other monomers copolymerizable with thealphaolefin.
 13. The laminate of claim 11 where in the polyolefin filmis a homopolymer of ethylene or propylene or a copolymer of ethylene andpropylene.
 14. A process for laminating two or more polyacrylonitrilefilms or a polyacrylonitrile film and a heat-sealable polyolefin filmcomprising:(a) incorporating into said polyacrylonitrile film anadhesion promoting proportion of a liquid heterized ehtyleneoxide-propylene oxide copolymer prepared by reacting said oxides with atleast one monohydroxy alcohol active hydrogen compound initiator havingfrom 1 to about 10 atoms; (b) interposing between said films a layer ofan adhesive composition comprising a combination of (1) astyrene-butadiene block copolymer having a monomer parts by weight ratioof from about 1:3 to about 3:1 and (2) a random terpolymer of from about30 to about 40 parts by weight of 1,3-pentadiene, about 30 to about 40parts by weight of a monocyclic terpene; and from about 25 to about 35parts by weight of alpha methylstyrene, said combination consistingessentially of from about 1 to about 3 parts by weight of the blockcopolymer to from about 3 to about 1 part by weight of the terpolymer;and (c) applying presure or pressure and heat to the film surfaces. 15.The process of claim 14 wherein said pressure is at least sufficient toform intimate interfacial contact between said films and said adhesivecomposition.
 16. The process of claim 14 wherein said adhesivecomposition is applied to one or both films from a solvent solution ofdispersion thereof, the solvent evaporated, interfacial contact made andthe laminate subjected to heat assisted bonding.
 17. A product pouch ortube fabricated from the flexible film laminate of claim 1.